Clenton E. Owensby

Long- and short-term effects of fire on nitrogen cycling in tallgrass prairie

Fires in the tallgrass prairie are frequent and significantly alter nutrient cycling processes. We evaluated the short-term changes in plant production and microbial activity due to fire and the long-term consequences of annual burning on soil organic matter (SOM), plant production, and nutrient cycling using a combination of field, laboratory, and modeling studies. In the short-term, fire in the tallgrass prairie enhances microbial activity, increases both above-and belowground plant production, and increases nitrogen use efficiency (NUE). However, repeated annual burning results in greater inputs of lower quality plant residues causing a significant reduction in soil organic N, lower microbial biomass, lower N availability, and higher C:N ratios in SOM. Changes in amount and quality of below-ground inputs increased N immobilization and resulted in no net increases in N availability with burning. This response occurred rapidly (e.g., within two years) and persisted during 50 years of annual burning. Plant production at a long-term burned site was not adversely affected due to shifts in plant NUE and carbon allocation. Modeling results indicate that the tallgrass ecosystem responds to the combined changes in plant resource allocation and NUE. No single factor dominates the impact of fire on tallgrass plant production.

FIRE AND GRAZING IN THE TALLGRASS PRAIRIE: CONTINGENT EFFECTS ON NITROGEN BUDGETS'

Fire and grazing occur together in many of the worlds grasslands, but their effects on nutrient cycling have usually been studied as if they acted separately. We hypothesized that grazing by large herbivores results in conservation of nitrogen that would otherwise be lost from burned grasslands. We tested this hypothesis in a series of experiments on burned and unburned tallgrass prairie grazed by cattle. We manipulated grazing using enclosures and mowing. Combustion losses of N from ungrazed plots (1.8 g.m-2 yr-1) burned in the spring were double those from similarly burned, grazed plots (0.9 g m-2 yr-1). These losses represented about half of the preburn, aboveground stocks of N. The magnitude of N loss was proportional to the standing crop biomass available for combustion. Fire temperatures and energy release were reduced by grazing. We used mowing to simulate locally heavy grazing in patches. In the absence of burning, mowing patches increased the likelihood that a patch would be regrazed and caused persistent reductions in the residual biomass remaining in a patch at the end of the growing season. Mowing did not influence patch utilization or residual biomass when pastures were burned. Thus, the effects of fire on grassland N budgets were modified by grazing, and the effects of grazing on the patch structure of grasslands were modified by fire. We conclude that accurately predicting volatile losses of nutrients from grassland ecosystems resulting from biomass burning may depend on understanding effects of grazing.

Long-term effects of annual burning at different dates in ungrazed Kansas tallgrass prairie.

Ungrazed tallgrass prairie plots in the Kansas Flint Hills have been burned annually at 4 different dates since 1928. Time of burning markedly altered the physiognomy and was the crucial factor effecting vegetation change. Late-spring burning, coinciding with emergence of the warm-season perennial grasses, increased grass production and favored Andropogon gerardii and Sorghas- trum nutans. Burning in winter, early-spring, or mid-spring reduced herbage production and shifted vegetational composition by differentially favoring other species. Andropogon scoparius increased with mid- and early-spring burning, while perennial forbs and sedges increased with early-spring and winter burning. Amorpha canescens was favored by all burning treatments. Mulch buildup in unburned, undisturbed plots increased Poa pratensis and tree species and eventually reduced grass production. The long-term effects of annual late-spring burning, even in dry years, was not detrimental to herbage production, species composition, or total basal cover in tallgrass prairie. Tallgrass prairie is fire-derived and fire-maintained (Stewart 1951). Historically, fires were intermittent and occurred at nearly any time of the year (Jackson 1965). In the 1880s, cattlemen observed that transient steers gained more weight on burned than on unburned range, and as a result, grazing leases later mandated annual burning (Kollmorgen and Simonett 1965). Time of burn- ing, however, was of little concern, and most pastures were burned in January or February to stimulate earlier greenup. Although voluminous literature exists on vegetational effects from fire, few studies have acknowledged the importance of time of burning. Aldous (1934) initiated preliminary research on burning ungrazed tallgrass prairie at different dates. Subsequent reports on herbage production (McMurphy and Anderson 1963) and botani- cal composition (McMurphy and Anderson 1965) were nonrepli- cated, short-term studies in which the unburned control was mowed and raked annually, and included data from years when the plots were not burned. This paper compiles earlier research and adds recent data from undisturbed control plots to evaluate the importance of time of burning on herbage production and species composition in ungrazed tallgrass prairie over the past 56 years.

Soil microbial response in tallgrass prairie to elevated CO2

Terrestrial responses to increasing atmospheric CO2 are important to the global carbon budget. Increased plant production under elevated CO2 is expected to increase soil C which may induce N limitations. The objectives of this study were to determine the effects of increased CO2 on 1) the amount of carbon and nitrogen stored in soil organic matter and microbial biomass and 2) soil microbial activity. A tallgrass prairie ecosystem was exposed to ambient and twice-ambient CO2 concentrations in open-top chambers in the field from 1989 to 1992 and compared to unchambered ambient CO2 during the entire growing season. During 1990 and 1991, N fertilizer was included as a treatment. The soil microbial response to CO2 was measured during 1991 and 1992. Soil organic C and N were not significantly affected by enriched atmospheric CO2. The response of microbial biomass to CO2 enrichment was dependent upon soil water conditions. In 1991, a dry year, CO2 enrichment significantly increased microbial biomass C and N. In 1992, a wet year, microbial biomass C and N were unaffected by the CO2 treatments. Added N increased microbial C and N under CO2 enrichment. Microbial activity was consistently greater under CO2 enrichment because of better soil water conditions. Added N stimulated microbial activity under CO2 enrichment. Increased microbial N with CO2 enrichment may indicate plant production could be limited by N availability. The soil system also could compensate for the limited N by increasing the labile pool to support increased plant production with elevated atmospheric CO2. Longer-term studies are needed to determine how tallgrass prairie will respond to increased C input.

Productivity, Respiration, and Light-Response Parameters of World Grassland and Agroecosystems Derived From Flux-Tower Measurements

Abstract Grasslands and agroecosystems occupy one-third of the terrestrial area, but their contribution to the global carbon cycle remains uncertain. We used a set of 316 site-years of CO2 exchange measurements to quantify gross primary productivity, respiration, and light-response parameters of grasslands, shrublands/savanna, wetlands, and cropland ecosystems worldwide. We analyzed data from 72 global flux-tower sites partitioned into gross photosynthesis and ecosystem respiration with the use of the light-response method (Gilmanov, T. G., D. A. Johnson, and N. Z. Saliendra. 2003. Growing season CO2 fluxes in a sagebrush-steppe ecosystem in Idaho: Bowen ratio/energy balance measurements and modeling. Basic and Applied Ecology 4:167–183) from the RANGEFLUX and WORLDGRASSAGRIFLUX data sets supplemented by 46 sites from the FLUXNET La Thuile data set partitioned with the use of the temperature-response method (Reichstein, M., E. Falge, D. Baldocchi, D. Papale, R. Valentini, M. Aubinet, P. Berbigier, C. Bernhofer, N. Buchmann, M. Falk, T. Gilmanov, A. Granier, T. Grünwald, K. Havránková, D. Janous, A. Knohl, T. Laurela, A. Lohila, D. Loustau, G. Matteucci, T. Meyers, F. Miglietta, J. M. Ourcival, D. Perrin, J. Pumpanen, S. Rambal, E. Rotenberg, M. Sanz, J. Tenhunen, G. Seufert, F. Vaccari, T. Vesala, and D. Yakir. 2005. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. Global Change Biology 11:1424–1439). Maximum values of the quantum yield (α  =  75 mmol · mol−1), photosynthetic capacity (Amax  =  3.4 mg CO2 · m−2 · s−1), gross photosynthesis (Pg,max  =  116 g CO2 · m−2 · d−1), and ecological light-use efficiency (εecol  =  59 mmol · mol−1) of managed grasslands and high-production croplands exceeded those of most forest ecosystems, indicating the potential of nonforest ecosystems for uptake of atmospheric CO2. Maximum values of gross primary production (8 600 g CO2 · m−2 · yr−1), total ecosystem respiration (7 900 g CO2 · m−2 · yr−1), and net CO2 exchange (2 400 g CO2 · m−2 · yr−1) were observed for intensively managed grasslands and high-yield crops, and are comparable to or higher than those for forest ecosystems, excluding some tropical forests. On average, 80% of the nonforest sites were apparent sinks for atmospheric CO2, with mean net uptake of 700 g CO2 · m−2 · yr−1 for intensive grasslands and 933 g CO2 · m−2 · d−1 for croplands. However, part of these apparent sinks is accumulated in crops and forage, which are carbon pools that are harvested, transported, and decomposed off site. Therefore, although agricultural fields may be predominantly sinks for atmospheric CO2, this does not imply that they are necessarily increasing their carbon stock.

Grazing management effects on plant species diversity in tallgrass prairie

Abstract A 6-year study was conducted in tallgrass prairie to assess the effects of grazing management (cattle stocking densities and grazing systems) on plant community composition and diversity. Treatments included sites grazed season-long (May to October) at 3 stocking densities (3.8, 2.8, and 1.8 hectares per animal unit), ungrazed control sites, and sites under a late-season rest rotation grazing system at this same range of stocking densities. Plant communities were sampled twice each season using a nearest-point procedure. Native plant species diversity, species richness, and growth form diversity were significantly higher in grazed compared to ungrazed prairie, and diversity was greatest at the highest stocking density. This enhancement of plant species diversity under grazing was not a result of increased frequency of weedy/exotic species. There were no significant effects of grazing system on plant diversity, nor any significant stocking density × grazing system interactions, indicating that animal density is a key management variable influencing plant species diversity and composition in tallgrass prairie and that effects of animal density override effects of grazing systems. Increasing cattle stocking densities decreased the abundance of the dominant perennial tall grasses, and increased abundance of the C4 perennial mid-grasses. The frequency of perennial forbs was relatively stable across grazing treatments. Abundance of annual forbs varied among years and grazing treatments. In half of the years sampled, annual forbs showed the highest frequency under intermediate stocking density. Patterns of responses among plant groups suggest that some species may respond principally to direct effects of grazers and others may respond to indirect effects of grazers on competitive relationships or on the spatial patterns of fuel loads and fires. Thus, this study suggests that large grazer densities, fire, and annual climatic variability interact to influence patterns of plant community composition and diversity in tallgrass prairie. Effects of varying management such as stocking densities and grazing systems on plant species diversity and the relative abundances of different plant growth forms or functional groups may have important consequences for grassland community stability and ecosystem function.

Carbon dynamics and microbial activity in tallgrass prairie exposed to elevated CO2 for 8 years

Alterations in microbial mineralization and nutrient cycling may control the long-term response of ecosystems to elevated CO2. Because micro-organisms constitute a labile fraction of potentially available N and are regulators of decomposition, an understanding of microbial activity and microbial biomass is crucial. Tallgrass prairie was exposed to twice ambient CO2 for 8 years beginning in 1989. Starting in 1991 and ending in 1996, soil samples from 0 to 5 and 5 to 15 cm depths were taken for measurement of microbial biomass C and N, total C and N, microbial activity, inorganic N and soil water content. Because of increased water-use-efficiency by plants, soil water content was consistently and significantly greater in elevated CO2 compared to ambient treatments. Soil microbial biomass C and N tended to be greater under elevated CO2 than ambient CO2 in the 5–15 cm depth during most years, and in the month of October, when analyzed over the entire study period. Microbial activity was significantly greater at both depths in elevated CO2 than ambient conditions for most years. During dry periods, the greater water content of the surface 5 cm soil in the elevated CO2 treatments increased microbial activity relative to the ambient CO2 conditions. The increase in microbial activity under elevated CO2 in the 5–15 cm layer was not correlated with differences in soil water contents, but may have been related to increases in soil C inputs from enhanced root growth and possibly greater root exudation. Total soil C and N in the surface 15 cm were, after 8 years, significantly greater under elevated CO2 than ambient CO2. Our results suggest that decomposition is enhanced under elevated CO2 compared with ambient CO2, but that inputs of C are greater than the decomposition rates. Soil C sequestration in tallgrass prairie and other drought-prone grassland systems is, therefore, considered plausible as atmospheric CO2 increases.

Assessment of C budget for grasslands and drylands of the world

Intergovernmental Panel on Climate Change (IPCC) estimates indicate that potential changes in seasonal rainfall and temperature patterns in central North America and the African Sahel will have a greater impact on biological response (such as plant production and biogeochemical cycling) and feedback to climate than changes in the overall amount of annual rainfall. Simulation of grassland and dryland ecosystem responses to climate and CO2 changes demonstrates the sensitivity of plant productivity and soil C storage to projected changes in precipitation, temperature and atmospheric CO2. Using three different land cover projections, changes in C levels in the grassland and dryland regions from 1800 to 1990 were estimated to be −13.2, −25.5 and −14.7 Pg, i.e., a net source of C due to land cover removal resulting from cropland conversion. Projections into the future based on a double-CO2 climate including climate-driven shifts in biome areas by the year 2040 resulted in a net sink of +5.6, +27.4 and +26.8 Pg, respectively, based upon sustainable grassland management. The increase in C storage resulted mainly from an increase in area for the warm grassland sub-biome, together with increased soil organic matter. Preliminary modeling estimates of soil C losses due to 50 yr of regressive land management in these grassland and dryland ecoregions result in a 11 Pg loss relative to current conditions, and a potential loss of 37 Pg during a 50 yr period relative to sustainable land-use practices, an average source of 0.7 Pg C yr−1. Estimates of the cost of a 20 yr rehabilitation program are 5 to 8×109 US

Effects of elevated CO2 and nitrogen fertilization pretreatments on decomposition on tallgrass prairie leaf litter

yr−1, for a C sequestering cost of approximately 10 US

Photosynthetic and Water Relations Responses to Elevated CO 2 in the C 4 Grass Andropogon gerardii

per tC.